Mark Green yesterday picked up the endorsement of Peter Vallone – a onetime supporter of Fernando Ferrer who urged the Bronx borough president to “throw the towel in” on becoming mayor.

Vallone warned that by not backing Green, Ferrer is helping Republican nominee Michael Bloomberg’s chances in next month’s general election.

Ferrer on Sunday withdrew his concession in Thursday’s runoff against Green – which Ferrer lost by about 20,000 to 25,000 votes, according to an unofficial count – and said he will wait for the official tally later this week.

“Any hesitation of endorsement, any stopping or slacking only gives the Republican opponent an unnecessary and an unfair advantage,” said Vallone, who had endorsed Ferrer in the runoff against after his own bid for City Hall flopped in the Sept. 25 primary.

Speaking to reporters outside City Hall, Vallone said that waiting until the Board of Elections certifies the vote isn’t necessary.

“I would throw the towel in until there is some proof – beyond what we know now – that there’s any way that that unofficial tally can be changed substantially,” he said.

For his part, Green took a more conciliatory approach and said Ferrer and his supporters should take their time.

“I am not worried that their delay will hurt New York. They should take all the time they need to think about what’s in their interests, the interests of the party, the interests of the city,” Green said during a campaign stop at a Jewish senior center in Briarwood, Queens.

Green also said “there’s no conceivable analogy” between last week’s runoff and the presidential vote count last year in Florida – a comparison made by some Ferrer backers.

“My 21,000-vote majority in one city is 40 times larger than [George W. Bush’s] 537-vote majority [over Al Gore] in Florida,” he said.

“I certainly think that at this moment he has disappointed many of Ferrer’s supporters.” Rivera said.

McCall had backed Ferrer against Green in last week’s runoff.

“I think clearly when there has been such unity among most major black and Latino leaders to at least see what would happen here and deal with the tenor of the Green campaign that there would have at least have been some dialogue before Carl McCall issued his endorsement,” Sharpton told The Post.